1. Field of The Invention
The invention relates to an internal broach for the internal broaching of profiles defined by a profile bottom and flanks, in particular of internal toothings, in a workpiece.
2. Background Art
The internal broaches conventionally used for the internal broaching of profiles are known from DIN 1415 (edition 1973), sheet 1, page 2. They comprise a shank, a toothed portion and an end portion. The shank is held by a puller of a broaching machine, which pulls the broach through a workpiece held in the broaching machine, thereby broaching the profiles. After the broaching operation, the end portion is seized by an end-portion holder of the broaching machine, which returns the broach to its initial position after the broaching operation. The toothed portion exhibits several rows of broach cutting teeth one after the other counter to the direction of broaching, as a rule a great number of broach cutting teeth. The broach cutting teeth comprise bottom-cutting edges for the machining of the bottom of a profile and flank-cutting edges for the machining of the flanks of the profile. The broach cutting teeth, which serve to cut a profile and are disposed one after the other counter to the direction of broaching, and which, in this regard, are allocated to each other, exhibit a depth stepping, i.e. ascending diameters, so that all the broach cutting teeth serving to machine a profile one after the other cut a chip serving to produce the bottom of the profile. Since the bottom-cutting edges deliver the main machining performance, they are also called main cutting edges. As far as the cutting of the flanks of the profile is concerned, the broach cutting teeth disposed one after the other counter to the direction of broaching exhibit flank-cutting edges that have a back taper as illustrated in DIN 1415 (edition 1973), sheet 1, page 3, picture 11. The flank-cutting edges are also called secondary edges. The back taper is produced in that, referred to the flank-cutting edges of a leading broach cutting tooth, the flank-cutting edges of a subsequent broach cutting tooth are provided with a lateral relief so that the flank-cutting edges of the subsequent broach cutting tooth only machine in the area provided by the ascending diameters or depth stepping, while not coming into engagement with the workpiece in the area where the flank-cutting edges of the leading broach cutting tooth have cut. This helps prevent any jamming of the broch cutting teeth in the vicinity of the flanks during the broaching operation. As a result, the flanks of the profile get a stepped surface structure.
The profiles produced by the known and conventional internal broaches have a surface quality and accuracy of profile shape and contour sufficient for normal applications and requirements. During the broaching operation, displacement of the axis of the broach may occur so that the broach cutting teeth that come into engagement one after the other each have a different central position relative to the workpiece to be broached. In particular during twist broaching (helical broaching), a torsion defect produced by rotatory forces during twist broaching may be superposed on such a displacement of the axis of the broach. Very often the accuracy of profile shape and the surface quality of the profile flanks are not sufficient, although the accuracy of flank contour will be satisfying as a rule. High accuracy of profile shape and contour is demanded in particular in the case of running gears, for instance internally toothed gears with spur or helical toothing.
In order to eliminate the deficiencies specified and to comply with correspondingly high requirements, it is known to provide the broach with a sizing portion subsequent to the back-tapered broach cutting teeth--referred to the direction of broaching. Such a sizing portion consists of several broach cutting teeth disposed one after the other, which are of identical height, not shaving the bottom of the profile. However, they have tooth thicknesses that increase counter to the direction of broaching, i.e. over the full height of the flank of the profile, all the sizing teeth cut a chip, the thickness of which generally amounts to 10 to 20 .mu.m. Each flank-cutting edge of the sizing teeth must be provided with a relief produced by grinding, i.e. a relief angle. Excellent accuracy of shape of the profile and high surface quality is obtained by sizing. As regards the accuracy of contour of the flanks, there is some deterioration as compared with the profile broached by depth stepping. This results from the fact that the relief-ground flank-cutting edges of the sizing teeth are sharp cutting edges, the self-guidance behavior of which is comparatively bad.
It is inherent in the system that the changeover from depth-stepped broaching to full-shape sizing is accompanied with an interruption of the broaching force that leads to considerable disadvantages in particular in the case of twist-broaching. Relieving the main cutting force that works counter to the direction of broaching will also lead to a reduction in torsional tension, i.e. the relative torsion between the workpiece and the internal broach changes. This change can be so strong that the full-shape-sizing edge does not correctly enter the profiles broached by depth stepping, as a result of which machining the flanks unilaterally so that the profile is not sized on both flanks. Owing to the deficiencies specified of the profile produced by depth-stepped broaching, the flank-cutting edges of the sizing teeth irregularly cut into the stepped flanks of the profile, this again generating torsional vibrations which negatively affect the accuracy of contour of the profile.